jarle/Bun DOM testing

DOM testing

Learn how to test DOM elements and components using Bun with happy-dom and React Testing Library

Bun's test runner plays well with existing component and DOM testing libraries, including React Testing Library and happy-dom.

happy-dom

For writing headless tests for your frontend code and components, we recommend happy-dom. Happy DOM implements a complete set of HTML and DOM APIs in plain JavaScript, making it possible to simulate a browser environment with high fidelity.

To get started install the @happy-dom/global-registrator package as a dev dependency.

bun add -d @happy-dom/global-registrator

We'll be using Bun's preload functionality to register the happy-dom globals before running our tests. This step will make browser APIs like document available in the global scope. Create a file called happydom.ts in the root of your project and add the following code:

import { GlobalRegistrator } from "@happy-dom/global-registrator";

GlobalRegistrator.register();

To preload this file before bun test, open or create a bunfig.toml file and add the following lines.

[test]
preload = ["./happydom.ts"]

This will execute happydom.ts when you run bun test. Now you can write tests that use browser APIs like document and window.

import { test, expect } from "bun:test";

test("dom test", () => {
  document.body.innerHTML = `<button>My button</button>`;
  const button = document.querySelector("button");
  expect(button?.innerText).toEqual("My button");
});

TypeScript Support

Depending on your tsconfig.json setup, you may see a "Cannot find name 'document'" type error in the code above. To "inject" the types for document and other browser APIs, add the following triple-slash directive to the top of any test file.

/// <reference lib="dom" />

import { test, expect } from "bun:test";

test("dom test", () => {
  document.body.innerHTML = `<button>My button</button>`;
  const button = document.querySelector("button");
  expect(button?.innerText).toEqual("My button");
});

Let's run this test with bun test:

bun test

bun test v1.3.3

dom.test.ts:
✓ dom test [0.82ms]

 1 pass
 0 fail
 1 expect() calls
Ran 1 tests across 1 files. 1 total [125.00ms]

React Testing Library

Bun works seamlessly with React Testing Library for testing React components. After setting up happy-dom as shown above, you can install and use React Testing Library normally.

bun add -d @testing-library/react @testing-library/jest-dom

/// <reference lib="dom" />

import { test, expect } from 'bun:test';
import { render, screen } from '@testing-library/react';
import '@testing-library/jest-dom';

function Button({ children }: { children: React.ReactNode }) {
  return <button>{children}</button>;
}

test('renders button', () => {
  render(<Button>Click me</Button>);
  expect(screen.getByRole('button')).toHaveTextContent('Click me');
});

Advanced DOM Testing

Custom Elements

You can test custom elements and web components using the same setup:

/// <reference lib="dom" />

import { test, expect } from "bun:test";

test("custom element", () => {
  // Define a custom element
  class MyElement extends HTMLElement {
    constructor() {
      super();
      this.innerHTML = "<p>Custom element content</p>";
    }
  }

  customElements.define("my-element", MyElement);

  // Use it in tests
  document.body.innerHTML = "<my-element></my-element>";
  const element = document.querySelector("my-element");
  expect(element?.innerHTML).toBe("<p>Custom element content</p>");
});

Event Testing

Test DOM events and user interactions:

/// <reference lib="dom" />

import { test, expect } from "bun:test";

test("button click event", () => {
  let clicked = false;

  document.body.innerHTML = '<button id="test-btn">Click me</button>';
  const button = document.getElementById("test-btn");

  button?.addEventListener("click", () => {
    clicked = true;
  });

  button?.click();
  expect(clicked).toBe(true);
});

Configuration Tips

Global Setup

For more complex DOM testing setups, you can create a more comprehensive preload file:

import { GlobalRegistrator } from "@happy-dom/global-registrator";
import "@testing-library/jest-dom";

// Register happy-dom globals
GlobalRegistrator.register();

// Add any global test configuration here
global.ResizeObserver = class ResizeObserver {
  observe() {}
  unobserve() {}
  disconnect() {}
};

// Mock other APIs as needed
Object.defineProperty(window, "matchMedia", {
  writable: true,
  value: jest.fn().mockImplementation(query => ({
    matches: false,
    media: query,
    onchange: null,
    addListener: jest.fn(),
    removeListener: jest.fn(),
    addEventListener: jest.fn(),
    removeEventListener: jest.fn(),
    dispatchEvent: jest.fn(),
  })),
});

Then update your bunfig.toml:

[test]
preload = ["./test-setup.ts"]

Troubleshooting

Common Issues

TypeScript errors for DOM APIs: Make sure to include the /// <reference lib="dom" /> directive at the top of your test files.

Missing globals: Ensure that @happy-dom/global-registrator is properly imported and registered in your preload file.

React component rendering issues: Make sure you've installed both @testing-library/react and have happy-dom set up correctly.

Performance Considerations

Happy-dom is fast, but for very large test suites, you might want to:

• Use beforeEach to reset the DOM state between tests
• Avoid creating too many DOM elements in a single test
• Consider using cleanup functions from testing libraries

import { afterEach } from "bun:test";
import { cleanup } from "@testing-library/react";

afterEach(() => {
  cleanup();
  document.body.innerHTML = "";
});